irmin-pack.unix: review usage of fsync and add where needed

[metanivek]

Let's review our usage of fsync in the new gc code and modify as needed.

• We can use the use_fsync repo config flag.
• Prior discussion on fsync for historical context: irmin-pack: add an option to force fsync on flush #886

[samoht]

From @icristescu and @metanivek discussion earlier today, a possible plan for crash consistency:

• call fsync after snapshot import;
• call fsync at the end of the gc, in the gc child process, to guarantee that all files created/updated by the gc are consistent
• the gc needs to copy the commit object in the prefix as well (currently it is the first object in the suffix).
• at startup if files are not in a consistent state:
  • check for the previous generation files;
  • ignore the suffix
  • reconstruct control file: from the mapping, and fstat dict.
  • restart the node from the commit in the prefix
• the guarantee is that if the first gc succeeds then we have always a set of files (prefix, mapping and dict) in a consistent state from which we can build the control file and start with a fresh suffix.

[metanivek]

the gc needs to copy the commit object in the prefix as well (currently it is the first object in the suffix).

will happen as a part of chunking the suffix. I moved the rest of the list to a new issue.